This invention relates to hydrophones and more particularly to a hydrophone which has an omnidirectional sensitivity characteristic over the widest possible frequency range.
Heretofore hydrophones have been made with piezoelectric materials in the form of rings, cylinders, plates, etc. These have been singularly poled either radially or in the axial or circumferential direction. If a tubular element is polarized through its thickness, i.e., radially poled, the sensitivity in the axial direction falls below the sensitivity in the radial direction when the diameter of the element is more than 5% of the wavelength of the sound. For elements poled in the axial or circumferential directions, the sensitivity in the radial direction falls below the sensitivity in the axial direction in the same range of diameter-to-wavelength. It has been determined that the directional sensitivity is largely due to sensitivity of the end caps and that a tubular element will remain omnidirectional up to a diameter-to-wavelength ratio of 0.5 if (1) the end caps are insensitive or if (2) the ends of the piezoceramic element are shielded from the sound field and the length of the tubular element is 1.5 times its diameter. This invention overcomes the problems of the prior art by combining in one piezoceramic tubular element a radially poled section with an axially or circumferentially poled section by which the end caps are made motionless if the element is driven electrically or insensitive to the sound field without shielding.